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An Annealing-Free ZnO:PEI Composite Cathode Interfacial Layer for Efficient Organic Solar Cells Chunyu Liu, Zhiqi Li, Xinyuan Zhang, Wenbin Guo, Liu Zhang, and Shengping Ruan ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01096 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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An Annealing-Free ZnO:PEI Composite Cathode Interfacial Layer for Efficient Organic Solar Cells Chunyu Liu1, Zhiqi Li1, Xinyuan Zhang1, Wenbin Guo1*, Liu Zhang2, and Shengping Ruan1 1

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering , Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China 2

College of Instrumentation & Electrical Engineering, Jilin University, 938 Ximinzhu Street, Changchun 130061, People’s Republic of China

ABSTRACT The polymer solar cells owning organic-inorganic composite cathode interlayer are fabricated to achieve well electron extraction compared to the devices with single inorganic semiconductor materials, leading to a significant enhancement of short-circuit current density (Jsc) and fill factor (FF) as well as slight improvement of open-circuit voltage (Voc). The incorporation of PEI mainly contributes to the decreased work function of ZnO layer and the reduced interfacial barrier, which are beneficial for the easy electrons extraction and the declined charge recombination. Strikingly, a post-annealing treatment is employed for the completed device with electrode to improve the interfacial characteristics between active layer and MoO3 layer, which is a successful method to further increase the Voc for the device based on PTB7:PC71BM. Our work provides a simple fabrication technology to improve the interfacial contact between active layer and adjacent interlayer, while possessing the major roles on the simultaneously enhanced Jsc, FF and Voc. KEYWORDS: ZnO:PEI composite interlayer, post-annealing treatment, open-circuit voltage, enhanced electron extraction, declined charge loss

Recently, the organic-inorganic composite interfacial layers have been widely employed into the research of organic solar cell (OSCs),1-4 which benefited from their combinative inherent features of excellent carrier extraction capacity for inorganic transition metal oxide and some special effects for organic materials on adjusting the work function (WF),5-8 facilitating electron coupling9 or passivating surface defects.10 Good conductivity, high transmittance, and smooth surface morphology are in particular essential for an eligible cathode interlayer in inverted OSCs, as well as a lower WF to extract more electrons from active layer.11,12 However, it is usually impossible for the common cathode interlayer with single material to satisfy all the points 1

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at once. ZnO, one kind of n-type semiconductor material, almost qualifies to serve as an excellent electron transport layer, but remains challenging because of nonnegligible charge recombination from interfacial barrier,13,14 which unquestionably blocks the charge transport and aggravates the charge loss. Recently, some organic polyelectrolyte materials, such as polyethylenimine (PEI),15 polyethylenimine ethoxylated (PEIE),16 and poly[(9,9-bis(3'- (N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene] (PFN),17 have been successfully used as cathode interlayer to decrease the electron transport barrier by reducing the WF of cathode, while being limited by their insulating nature and low conductivity. Meanwhile, the device performance is sensitive to the thickness of such cathode buffer layer, which is usually controlled within about 10 nm. Furthermore, a smart way has once been put forward to employ dual cathode interlayers, which utilized a polyelectrolyte to modify the metal oxide layer.18,19 Indeed, the interface characteristics between active layer and metal oxide were re-modulated, reducing or even eliminating the interfacial electron transport barrier. About 30% enhancement of power conversion efficiency (PCE) has been presented in the previous reports, demonstrating the certain effect on the improved device performance.20-22 However, the inherently insulating property of polyelectrolyte still exists and inevitably increases the series resistance of devices. In the meantime, the fabrications of these two films and such a thin film for polyelectrolyte are relatively complex and difficult for massive production of printed OSCs in the future. Herein, a simple and effective approach was paved by adopting annealing-free ZnO:PEI composite cathode interlayer and a high efficiency of 9.31% was achieved. The ZnO:PEI used here circumvents the problems of UV-ozone treatment for ITO substrate before spin-coating and thermal annealing treatment for fabricated ZnO:PEI film. There have been some reports proved that the UV-ozone treatment will increase the WF of ITO,23,24 which is reverse for electron collection of ITO electrode in inverted solar cells, and the resistance of ITO will increase after thermal annealing treatment,25 seriously affecting the device performance. The introduction of PEI could adjust the WF of composite interlayer compared with the pristine ZnO layer, reducing the interfacial barrier and charge recombination. At the same time, the conductivity, optical transmission, and film morphology of ZnO will not be significantly damaged by a low concentration of PEI doping. It can be concluded that the improved short-circuit current density (Jsc) and fill factor (FF) are mainly attributed to the enhanced electron extraction capacity because of the adoption of ZnO:PEI composite interlayer. A slightly increase of open-circuit voltage (Voc) from 0.737 to 0.747 V was also observed, which arose from the decreased charge loss due to reduced charge transport barrier. As everyone knows that the PTB7:PC71BM layer requires no 2


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annealing process, distinguishing from other active layer materials. 26,27 However, a surprising phenomenon is strikingly noticed that another further improved Voc of 0.760 V is achieved by post-annealing the optimal device, which can be attributed to the improved interfacial characteristics between active layer and MoO3 layer. Eventually, a champion performance of 9.90% was attained with a Voc of 0.760 V, a Jsc of 17.80 mA/cm2, and a FF of 73.15%.

EXPERIMENTAL SECTION The devices were fabricated with the structure of indium tin oxide (ITO)/ZnO:PEI/PTB7:PC71BM/ MoO3/Ag, the ZnO:PEI was treated as composite cathode interlayer to improve the electron extraction capacity. The fabrication processes of devices are schematically shown in Figure 1. The synthesis of annealing-free ZnO has been provided in our previous report.28 Next, a 0.04, 0.08, and 0.12 mg PEI was added into 1.0 mL ZnO solution to obtain the mixed solutions, respectively. The ZnO and ZnO:PEI solutions were spin-coated onto pre-cleaned ITO substrate at 4000 rpm for 40 s. After a 20 min solvent evaporation, PTB7:PC71BM (1:1.5 by weight) was spin-coated on ZnO and ZnO:PEI layers from a cosolvent of chlorobenzene:1,8-diiodooctane (97:3 by volume) solution at 1000 rpm for 60 s. Subsequently, all the devices were completed by thermally evaporating MoO3 (10 nm) and Ag (100 nm) under a low vacuum (5×10-4 Pa), respectively. The ZnO mixed with 0.04, 0.08, and 0.12 mg/mL PEI were indexed as ZnO:PEI-1, ZnO:PEI-2, and ZnO:PEI-3. The devices with pristine ZnO and ZnO doped with 0.04, 0.08 and 0.12 mg/mL PEI buffer layers were named as Device A to D. In addition, the post-annealing treatment for the prepared Device C with the optimal doping concentration of 0.08 mg PEI was carried out. The effective area is ca. 0.044 cm2.

Figure 1. The fabrication processes of OSCs devices.

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RESULTS AND DISCUSSION To ensure the sufficient light absorption of active layer, the high optical transmission of cathode interlayer is a pivotal factor in the inverted OSCs.29 Herein, the light transmission spectra of ITO substrate, ITO/ZnO, and ITO/ZnO:PEI were measured and displayed in Figure S1. It can be found that the ITO/ZnO exhibits similar transmission ratio that is comparable to ITO substrate. Meanwhile, the ZnO:PEI composite layers also demonstrate ignorable changes and allow majority photons to reach the active layer with the low doping concentration of PEI. To investigate the surface morphology of ZnO and ZnO:PEI films, the atomic force microscope (AFM) images were measured and displayed in Figure 2. Seen from the Figure 2a, the pristine ZnO film presents uniform NPs and relatively lower root mean square roughness (Rq) of 3.18 nm. While for the ZnO:PEI composite layer, the Rq are gradually increased with the varying doping concentration of PEI, which could be explained by the inadequate mixing of organic and inorganic phases. Thus, in order to obtain a higher efficiency of OSCs, the doping concentration of PEI should be limited in a relatively low level.

Figure 2. The AFM images of (a) pristine ZnO film, (b) ZnO:PEI-1, (c) ZnO:PEI-2 and (d) ZnO:PEI-3.

Figure 3a is the current density-voltage (J-V) characteristics of devices with ZnO and ZnO:PEI composite layers, and the corresponding performance parameters are listed in Table 1. The Device C demonstrates the best performance of 9.31%, with the improved Jsc from 16.02 to 17.15 mA/cm2 and FF from 67.44 to 72.70%. The Voc exhibits a little change from 0.737 to 0.747 V. The enhanced device performance mainly ascribes from the facilitated electron extraction capacity of ZnO:PEI. However, it can be found the Device D with the PEI of 0.12 mg/mL begin to degenerate, which originates from the decreased conductivity of ZnO:PEI composite interlayer. It can be understood that the incorporation of the PEI inevitably decreases the conductivity of ZnO:PEI composite film due to the insulation property of PEI. To prove this conclusion, the current-voltage (I-V) curves of the devices for ITO/ZnO and ZnO:PEI/Ag were measured in dark and shown in Figure 3b. It behaves the 4


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gradually decreased current with the increase of doping concentration. Figure 3c shows the typical incident photon-to-current efficiency (IPCE) spectra of all devices to identify the enhanced Jsc. The maximum IPCE value reaches to 77%, which is higher than that of control device (about 70%). Meanwhile, the overall improvements from 350 to 700 nm were achieved compared with device based on the pristine ZnO layer, suggesting that more photons in this range were converted into the effective charge carriers.30,31 This can be attributed to the decreased charge recombination and facilitated charge transport by reducing interfacial barrier, resulting in the increase of Jsc.32,33 Furthermore, the devices based on the PCDTBT:PC71BM were also fabricated to examine the universal applicability of ZnO:PEI layer, and the corresponding J-V curves are shown in Figure S2. It can be observed that an obviously enhanced device performance is provided from 5.70% to 6.69% by using ZnO:PEI composite interlayer.

Figure 3. (a) The J-V characteristics of devices with ZnO layer and ZnO:PEI composite layers, (b) the dark I-V characteristics for the device of ITO/ZnO or ZnO:PEI/Ag, and (c) the corresponding IPCE curves of all fabricated devices.

Table 1. The Photovoltaic Parameters of Devices with ZnO Layer and ZnO:PEI Composite Interlayer. Device

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

A

0.737±0.001

16.02±0.11

67.44±0.12

7.96±0.08

B

0.743±0.001

16.33±0.09

69.55±0.15

8.44±0.08

C

0.747±0.001

17.15±0.10

72.70±0.14

9.31±0.09

D

0.749±0.001

16.74±0.09

71.78±0.14

9.00±0.07

The photovoltaic parameters are the average values from 30 identical devices for each type.

The improved Jsc and FF can be attributed to the enhanced electron extraction capacity of ZnO:PEI layer, which could facilitate electron transfer and decrease the charge recombination at the interface. To testify the enhanced electron extraction capacity of ZnO:PEI, the photoluminescence (PL) measurements of PTB7 spin-coated on ZnO and ZnO:PEI were conducted and shown in Figure 4. It comes to light that the PL emission of one material will be quenched if it transmits charge to another material.34,35 In Figure 4, the PTB7 film on the ZnO:PEI displays partly quenched PL intensity with a emission peak at about 720 nm compared with PTB7 on

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the ZnO, indicating that more electrons were transferred to ZnO：PEI layer. That is to say the ZnO:PEI composite interlayer possesses stronger capacity to extract electrons.

Figure 4. The PL spectra PTB7 covering on the ZnO and ZnO:PEI composite layers.

The enhanced electron extraction capacity derives from the decreased interfacial barrier against electron transport due to the reduced WF of ZnO:PEI interlayer. It has been widely reported that the PEI could decrease the WF by modifying the surface of material. In order to explore the effect of doping on WF of ZnO:PEI composite film, the Kelvin probe were carried out and the results were arranged into Figure S3. It can be noted that the WF was reduced with the addition of the PEI. The energy levels diagram of PC71BM and ZnO:PEI layer are shown in Figure 5a. As shown in Figures 5b and 5c, the unbalanced WFs will lead to the energy band bending and generate the unexpected interfacial barrier against electron transport. The ZnO:PEI with lower WF reduces the interfacial barrier for electron transport compared with ZnO, which is beneficial to enhance electron extraction from active layer and decrease interfacial charge recombination.

Figure 5. (a) The energy level of PC71BM with ZnO and ZnO:PEI, the energy band bending of PC71BM contacting with (b) ZnO and (c) ZnO:PEI.

To realistic estimate the facilitated charge extraction in the whole device, the technology of injected charge 6


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extraction by linearly increasing voltage (i-CELIV) was employed. The Figure 6a and 6b are i-CELIV current transients with a small offset voltage (Uoffset) of -0.6 V for Device A and C at different reverse bias voltage (Umax). The Uoffset drives the charge carrier into device, while the Umax is applied to extract them.36 It can be found that maximum of the extraction current (Imax) increases with the variation of the Umax from 1 to 8 V. While the time (Tmax) reaching the Imax gradually shifts to a shorter value. It is easy to understand that the trapping state in device could be gradually filled as Umax increases, which could improve the diffusion rate of carrier and boost the extraction process of charge carriers.37,38 Figure 6c displays △I/I0 (△I=Imax-I0) under different Umax for Device A and C, where I0 is the displacement current. The △I/I0 of Device C is always higher than that of Device A, suggesting that it is toilless to extract charge in Device C. Figure 6d shows the corresponding Tmax as a function of Umax. The Tmax of Device C was decreased by employing ZnO:PEI interlayer, mainly originating from the different ability of charge extraction.37 The incorporation of PEI into ZnO could reduce the interfacial barrier, facilitating charge extraction and decreasing the charge recombination, thus the more carriers will be more quickly extracted than Device A based on the pristine ZnO interlayer.

Figure 6. The i-CELIV current transients of (a) Device A and (b) Device C recorded as a function of Umax. (c) ∆I/ I0 and (d) Tmax for the Device A and C recorded at different Umax.

After the ZnO:PEI composite interlayer was used, the Voc was slightly enlarged from 0.737 to 0.747 V, which can be put down to the decreased charge loss because of the well re-modulated energy level alignments. There have been some reports that the Voc is sensitive to the energy levels of materials as well as the engineering of 7


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interfaces and contacts.39,40 Furthermore, the ideal ohmic contacts are respected to decrease the charge loss,41 while some behaves (energy level offsets and band bending) at non-ideal contacts due to the Fermi-level differences will inevitably cause a decreased Voc.42,43 In other words, the deliberately adjusted energy level alignments and the carefully modified interfacial contact are of importance to ensure a high Voc. In this work, the WF of ZnO:PEI is intentionally reduced as shown in Figure S3, which decreases the charge loss caused by the energy level offsets and band bending, leading to a weakly enlarged Voc. Figure 7 plots the J-V characteristics of Device C without and with post-annealing treatment for the whole prepared devices at 50, 60 and 70 °C, and the detail photovoltaic parameters have been given in Table S1. It is a matter of concern that the Voc was further improved from 0.747 to 0.760 V after post-annealing at 60 °C for 10 min compared to Device C without any treatments, accompanying with a higher PCE of 9.90 %. We can speculate that the improved Voc arises from some positive effects on the interface contact between adjacent layers after the post-annealing treatment. In order to verify this, Device A post-annealed at the same conditions after fabricating ZnO interlayer, PTB7:PC71BM film and MoO3 layer are respectively prepared. The selection of Device A is to exclude the effect of PEI. We found that the performance and Voc of the first two devices can’t be improved. While, a significant enhancement of Voc was observed when the post-annealing treatment was carried out after evaporating MoO3 layer, as shown in Figure S4, and the corresponding photovoltaic parameters are also provided in Table S1. The MoO3 layer is prepared by thermally evaporating, so there is minimal impact on the material itself from thermal annealing. Thus, we deduce that the post-annealing mainly influences the interfacial contact between active layer and MoO3 layer, facilitating the micro contact between them. The post-annealing for the MoO3 layer relatively complicates the fabrication technology, thus the devices are still fabricated by post-annealing treatment after evaporating Ag electrode. In the subsequent study, the Voc will be treated as the research emphasis, which is a meaningful topic for the research of OSCs based on PTB7:PC71BM. The corresponding IPCE spectra of Device C with post-annealing treatment are given in Figure S5, which are approximately consistent with the result of J-V characteristics.

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Figure 7. The J-V characteristics of Device C without and with post-annealing treatment.

In our previous discussion, we have investigated the improved Voc by some profile analysis, and the improved Voc by employing ZnO:PEI interlayer and post-annealing treatment will be simultaneously examined in the following research. The J-V characteristics in dark for Devices A and C without and with post-annealing treatment are measured and displayed in Figure 8. Compared with Device A, the reverse saturation current of Device C without and with post-annealing treatment are remarkable decreased, which could contribute to the increase in Voc.11,44 In the forward bias region, it displays a larger turn-on voltage, which suggests the increased built-in potential (Vbi) across the device, resulting in the improvement of Voc.45,46 Meanwhile, the Device C without and with the post-annealing treatment demonstrate larger rectification effect, reducing the limitation from shunts for device performance.47

Figure 8. The J-V characteristics of Device A and Device C without and with post-annealing treatment.

Additionally, the i-CELIV current transients under different number of injection charge are also measured to further verify the decreased charge loss. Figure 9 shows the i-CELIV current transients of Device A and C 9


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without and with post-annealing treatment under various Uoffset from -0.1 to -1.0 V, recorded at Umax=5 V. By applying a Uoffset that drives the charge carrier towards the device, a sheet of injection charge can be generated and extracted.48 When the Uoffset is lower, the injected charge will be trapped by the internal defect states or recombined due to the blocking of interfacial transport barrier, thus the extraction current can’t be obviously observed. It can be noted that there is a significant charge extraction when the Uoffset reaches to -0.5 V for the Device A, while the latter two exhibit the visible extraction current envelope at the Uoffset =-0.4 V, indicating that the injected charge carrier can be more easily extracted instead of being recombined.49 The devices based on ZnO:PEI interlayer and post-annealing treatment could reduce the charge loss, which conduces to the enhanced Jsc and Voc.

Figure 9. The i-CELIV current transients of (a) Device A, Device C (b) without and (c) with post-annealing treatment recorded as a function of Uoffset.

CONCLUSIONS In summary, a high performance of 9.90% for OSCs was attained by employing ZnO:PEI composite cathode interlayer and the post-annealing treatment. Moreover, an attractively enlarged Voc was demonstrated by post-annealing treatment after thermally evaporating MoO3 and Ag electrode for the device based on PTB7:PC71BM. The test results of i-CELIV have ultimately revealed that charge extraction ability in the optimized device is remarkably enhanced. The Jsc and FF are significantly improved due to facilitated electrons extraction of ZnO:PEI interlayer and smoother charge transfer arising from the reduced interfacial barrier. Meanwhile, the improved Voc can be attributed to the decreased reverse saturation current and improved Vbi, accompanying with the declined charge loss from interfacial recombination. These methods are of great significance for the modulations of interfacial properties, judiciously improving charge carrier extraction characteristics in the whole device.

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ASSOCIATED CONTENT Supporting Information The transmission spectra, WF and additional J-V characteristics, IPCE and photovoltaic parameters presented in the manuscript. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: W. B. Guo, [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to National Natural Science Foundation of China (61370046, 11574110), the Science and Technology Innovation Leading Talent and Team Project of Jilin Province (20170519010JH), International Cooperation and Exchange Project of Jilin Province (20170414002GH), and Project of Graduate Innovation Fund of Jilin University (2017175, 2017170) for the support to the work.

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